One of the most important and difficult components in a wide Field Of View (FOV) optical sensor is the beam steering mechanism, or “Line Of Sight (LOS) toggle.” Optical sensors, especially high resolution wide FOV sensors, often rely on composite imagery to generate a complete image because imaging circuitry from Focal Plane Arrays (FPAs) and other electronic components of the sensor require too much space to generate a single continuous image in one data capture. Accordingly, FPAs are often arranged so that each complete image (frame) consists of two or more data captures taken in rapid succession.
In these composite imaging systems, each data capture covers an equal portion of the wide FOV such that, when all data captures associated with a frame are put together, a complete image is formed. However, for working of composite imagery systems, the optical sensor system must be capable of rapidly switching between multiple Fields Of Regard (FORs). This switching must occur at least twice per frame, one for each of the FOR. For example, if the composite image was designed to compile two data captures to create a complete frame, the optical sensor system would need to adjust lens FOR orientation twice per frame. This means that a video frame rate of 72 frames per second would require 144 mechanical adjustments of highly sensitive lenses per second, and some military applications exceed this frame rate.
The traditional method of switching between FORs is to use a Risley prism pair. These devices use two wedged lenses, which are then rotated relative to one another in order to alter the bending angle of light passing through the Risley prism, thereby adjusting the FOR. However, Risley prisms require heavy, complex mechanical drives to provide the circumferential rotation necessary to switch between FORs. Also, Risley prisms are inefficient for high-speed FOR switching because a two-mode Risley prism requires a full 180 degree rotation for each switch, requiring excessive motion and energy to operate.
Another known method of scanning multiple FORs is a “ball and socket” device which uses two substantially equal spherical lenses arranged so that the convex side of one lens mates with the concave side of the second lens. This device also requires that a thin layer of lubricant be deposited between the two lens surfaces to maintain image quality and functionality. Unfortunately, this simple spherical design results in both spherical and chromatic aberrations, which are undesirable. Furthermore, some wide FOV imaging systems require that the optical components operate within a vacuum, making lubricants, which tend to outgas and thereby ruin a vacuum atmosphere, an unacceptable component of the FOR scanning mechanism used in outer space system.
A need therefore exists for a vacuum-compatible optical LOS toggle capable of providing aberration corrected, high quality images while minimizing the size of mechanical drive machinery and limiting movement of the optical components.